Reply by MrRon

“For clarification purposes, I’m attaching a 21 inch red oak 2×4 flat against some 5/4 red oak with glue between the joint. And it needs to be very sturdy, as I’d like the quality of this piece to be high and lasting.”

Are both sides exposed to view? Best to fasten through the 5/4 into the 2×4 and if exposed, counterbore and plug.